One type of a physical quantity sensor is an angular rate sensor. As this sort of angular rate sensors, there are angular rate sensors generally manufactured by employing semiconductor substrates. In the general-purpose angular rate sensors, such a sensor is proposed in which a movable unit is constituted by a drive-purpose vibration unit which is vibrated along a first direction, and a detection-purpose vibration unit which is vibrated along a second direction perpendicular to the first direction by Coriolis force. Here, the Coriolis force is generated by applying an angular velocity to the sensor, when the drive-purpose vibration unit is vibrated along the first direction. For example, it is disclosed in Japanese Patent Application Publication No. 2001-91265, which corresponds to U.S. Pat. No. 6,450,033.
In such an angular rate sensor, the movable unit is formed so as to be released from a supporting substrate by etching a semiconductor layer supported by the supporting substrate. For instance, there is proposed an angular rate sensor employing an SOI (silicon-on insulator) substrate made by adhering both silicon substrates via an oxide film to each other. For example, it is disclosed in Japanese Patent Application Publication NO. 2001-133268.
The angular rate sensor corresponds to a front surface processed type angular rate sensor. In this sensor, while one of the silicon substrates for constructing the SOI substrate is employed as a supporting substrate, the movable portion is formed on the other silicon substrate by executing a well-known micro-machine processing technique such as a trench etching process and a sacrificial layer etching process with respect to both the other silicon substrate and the oxide film from the front surface side of the other silicon substrate.
Also, in the front surface processed type angular rate sensor (e.g., yaw rate sensor), in order to perform the etching process in a high efficiency and to reduce the movable unit in weight, a plurality of through holes is formed in a large area portion in such as an etching remaining portion of the movable unit. For example, it is disclosed in Japanese Patent Application Publication No. 2001-99861, which corresponds to U.S. Pat. No. 6,450,029.
On the other hand, in the angular rate sensor having such a movable unit, the movable unit released on the supporting substrate is displaced within, for example, the horizontal plane along the layer plane of the semiconductor layer which constitutes the movable unit.
In particular, while the movable unit is constituted by both the drive-purpose vibration unit and the detection-purpose vibration unit, the drive-purpose vibration unit is vibrated along the first direction by drive means, or the like, whereas the detection-purpose vibration unit is vibrated along the second direction perpendicular to the first direction by applying thereto the angular velocity when this drive-purpose vibration unit is vibrated along the first direction.
In the angular rate sensor having both the drive-purpose vibration unit and the detection-purpose vibration unit as the variable unit, plural through holes are formed so as to penetrate through the movable unit along the thickness direction of the semiconductor layer in order to improve efficiency of the etching process and to reduce the movable unit in weight.
However, in the conventional angular rate sensor, since the patterns of these through holes are provided by considering only the uniformity of the etching, the patterns of the through holes to be formed are same in both the drive-purpose vibration unit and the detection-purpose vibration unit.
As a typical pattern of through holes in a related art, as indicated in FIGS. 4A and 4B, such arrayed patterns are employed in both a drive-purpose vibration unit 20 and a detection-purpose vibration unit 30, in which longitudinal directions of through holes 20a having rectangular shapes are aligned with each other along the same direction.
In the case that the arrayed pattern of the through holes 20a formed in the drive-purpose vibration unit 20 are identical to those formed in the detection-purpose vibration unit 30, attenuation of vibrations, namely damping of vibrations is produced due to an influence of eddies of air generated in opening edge portions of the through holes 20a while the vibration unit is vibrated, depending upon a vibration direction of the vibration unit.
For instance, in the through hole patterns shown in FIGS. 4A and 4B, the drive-purpose vibration unit 20 is vibrated along an x direction as a first direction, whereas the detection-purpose vibration unit 30 is vibrated along a Y direction as a second direction. As to the arrayed pattern of the through holes 20a, when the detection-purpose vibration unit 30 is vibrated along the Y direction, the influence of the damping is increased.
If the damping happens to occur in both the vibration units, then losses of vibration energy are produced due to this damping. Accordingly, there is a risk that the vibrations become unstable. It is apparent that this risk may give an adverse influence to the sensor characteristic.
In order to avoid the damping caused by the air resistance, and the like, the conventional angular rate sensors are maintained under vacuum condition. To this end, the entire structure of a conventional angular rate sensor is sealed by a vacuum package. However, this structure may increase the manufacturing cost of the conventional angular rate sensors, which is not desirable.
Further, conventionally, a mechanical quantity sensor is constituted by a supporting substrate, a movable unit, and a peripheral fixing unit. The movable unit is supported on the supporting substrate under such a condition that the movable unit is separated from the supporting substrate to be arranged. The peripheral fixing unit is arranged at a peripheral portion of the movable unit on the supporting substrate, and is fixed to the supporting substrate so as to be supported.
In this mechanical quantity sensor, when a mechanical quantity is applied to this sensor, the movable unit can be displaced along the horizontal direction with respect to the plane of the supporting substrate. Then, this mechanical quantity sensor detects the applied mechanical quantity based upon the displacement state of the movable unit when the mechanical quantity is applied thereto.
As such a mechanical quantity sensor, for instance, an angular rate sensor which is formed by processing a semiconductor substrate is proposed. For example, it is disclosed in Japanese Patent Application Publication No. 2001-133268,
In such a mechanical quantity sensor, when this sensor is operated, a constant potential is applied to the movable unit and an AC voltage is applied to a driving electrode which is provided opposite to the movable unit so as to displace the movable unit. In other words, the movable unit is driven by utilizing electrostatic attracting force in the conventional mechanical quantity sensor.
In the mechanical quantity sensor, the supporting substrate separately arranged under the movable unit is brought into the GND state when the sensor is operated. As a result, when the mechanical quantity sensor is operated, a large potential difference is produced between the movable unit and the supporting substrate. Therefore, there is a high possibility that the movable unit may be adhered to the supporting substrate due to the electrostatic attracting force, namely, a so-called “sticking” phenomenon may occur.